JP2004-183746A discloses a conventional device in which a flow control valve is provided in a discharge side flow passage of an oil pump.
In JP2004-183746A, a spool that functions as the flow control valve includes a orifice, and in accordance with a differential pressure between an upstream side and a downstream side of the orifice, the spool moves, thereby adjusting an opening area of a drain hole that is provided in a side face of the spool so as to communicate with an intake passage of the oil pump. The spool is normally biased to close the drain hole by a spring or the like, but when the differential pressure between a pressure on the upstream side of the orifice and a pressure on the downstream side of the orifice exceeds a biasing force of the spring, the spool moves so as to open the drain hole.
By using this flow control valve to return working oil discharged from the oil pump to the intake passage of the oil pump, a reduction is achieved in an intake amount of working oil intermixed with air having low compressibility, and as a result, a discharge efficiency of the oil pump improves.
However, when an increase occurs in an amount of air mixed into working oil suctioned into the oil pump from an oil pan, a discharge pressure of the oil pump pulsates, leading to pulsation in the differential pressure. When this pulsation causes the differential pressure to decrease in a case where the drain hole is required to be in a communicative state, the spool may be moved so as to close the drain hole by the biasing force of the spring, and as a result, the drain hole may enter a non-communicative state.
In such cases, working oil intermixed with air having low compressibility is suctioned into the oil pump, leading to an increase in a volume of air contained in the oil in the oil pump, and as a result, the discharge pressure of the oil pump decreases further, making it impossible to supply an oil pressure required by a transmission or the like, for example.
In response to this problem, JP2002-340160A discloses a device that determines the amount of air mixed into the working oil and increases a rotation speed of an electric oil pump in consideration of a reduction in the discharge pressure caused by the intermixed air.